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Per definition, an Autonomous System (AS) is a network (or entity) sharing the same rules on how it should connect to the rest of the internet (routing policy). As such, it functions as one independent sub-section of the internet. This is done to make internet routing more comprehensible. Each AS is assigned a specific number (ASN) that can be used in BGP (Border Gateway Protocol) routing, defining paths for traffic to follow. In most cases, holders of ASN have multiple uplinks where traffic can be diverted in case of a failure. As such, no single point of failure can bring down the entity. Just like with IP-networks, one big ASN can have several smaller ones behind it. This makes global routing scalable, as other networks only have to keep track of the larger ASN, functioning as a front for the rest. Not all connections are public, as some have private peering (exchange of data between two or more ASN), that is not visible in the public route-view servers. This is an exception that may warrant having an ASN, even though you are not multihomed (and thus most likely share the general routing policy of your uplink).

In short, BGP is the communication tool used to relay information between different networks or groups of networks (defined by ASN). These different sections are assigned IP-networks (or prefixes) in the routing table. As such, BGP can be used to define what routes traffic should go through, and under what circumstances (following a list of set rules, or attributes). In essence, BGP defines reachability/preferred paths across different networks or groups of networks. Exchanging routing information between internet service providers (ISPs) is essential for maintaining global traffic paths. In some really large networks, BGP can be used internally within the same AS. However, it is most commonly used externally between different AS.

Each router will have a routing table, listing prefixes of all its neighbors (or peers). This makes it possible for routers to know what path to follow. Since the introduction of BGP4, efforts are made to decrease the size of routing tables. The primary solution implemented since version 4, is classless inter-domain routing (CIDR), explained in the applicable section. Note that the routers within each ASN in the scheme below, may still communicate using IBGP, which is used solely for internal communication. BGP is primarily used between edge routers (the front routers of each network, defined by ASN).


|-----------------|                |---------------------------------|  ← IBGP → |  internal router A |

|     AS 1        |     ← BGP 4 →  |    edge router of AS 2          |           |--------------------|

|-----------------|                |---------------------------------| ← IBGP → |-------------------|

                                                                                | internal router B |


By entering an ASN above, you will get detailed information on the connectivity of said autonomous system, where you will be able to follow traffic routes through different sections of the internet and you will be presented with a list of prefixes originating from that AS-number.